FabFilter Pro‑L Help
Most of the time, choosing the right preset from FabFilter Pro-L's preset menu works very well. But in some cases, you might want to adjust and fine-tune the limiting behavior. To access Pro-L's advanced settings, click the Advanced button right under the Gain slider at the left of the interface, which makes the advanced panel slide out. Clicking the button again will hide the panel.
FabFilter Pro-L comes with four advanced and highly program-dependent limiting algorithms, which all have their own distinct character. One is not better than the other; they are all great, and they all go very loud if you want them to.
It is best to select the appropriate algorithm depending on what kind of effect you want to achieve: some are designed to be as transparent as possible, while others may add a nice punch or flavor. Here's a short description of all algorithms:
The Lookahead knob sets the look-ahead time for the initial 'transient' stage. This allows the limiter to examine the incoming audio in advance and predict the amount of gain reduction needed to meet the requested output level. If the look-ahead time is very short, the limiter doesn't have much time to move to the desired level: this will generally have the effect of preserving transients better and increasing the apparent loudness, but at the expense of possible distortion. Longer look-ahead times are safer, but less loud, and transients might appear to get lost in the mix a little more.
Very short look-ahead times (less than 0.1 ms) will approximate 'hard clipping', introducing distortion and aliasing. This causes inter-sample peaks which can cause further distortion later on. To reduce aliasing and inter-sample peaks, we advice to use oversampling. Also, turn on ISP metering (also known as True Peak metering) to visualize the inter-sample peaks generated. This enables you to adjust the output level accordingly.
Apart from the fast 'transient' stage, the limiter has a slower 'release' stage that responds to the overall dynamics of the incoming audio. The Attack and Release knobs control how quickly and heavily the release stage sets in. Shorter attack times will allow the release stage to set in sooner; longer release times will cause it to have more effect.
In general, short attack times and long release times are safer and cleaner, but they can also cause pumping and reduce clarity. On the other hand, long attack times and short release times can increase apparent loudness and presence, but at the expense of possible distortion.
When limiting a stereo signal, it is generally desirable to process both channels in the same way to avoid changing the stereo image inadvertently. However, when a short peak occurs in one channel, removing it is often almost inaudible. In this case, it is better to remove it only in the channel where it occurs.
You can control this behavior completely with the two channel linking knobs. The Transient knob controls the amount of channel linking for the 'transient' stage that mainly operates on short peaks. It often works well to choose less than 100% here. The Release knob controls the channel linking for the 'release' stage, where it is best to start with 100% which will completely link the channels. However, you can of course experiment with different settings depending on the level of limiting and the character of the incoming audio signal.